Electronic device with secondary battery

ABSTRACT

In the case where a film, which has lower strength than a metal can, is used as an exterior body of a secondary battery, a current collector provided in a region surrounded by the exterior body, an active material layer provided on a surface of the current collector, or the like might be damaged when force is externally applied to the secondary battery. A secondary battery that is durable even when force is externally applied thereto is provided. A cushioning material is provided in a region surrounded by an exterior body of a secondary battery. Specifically, a cushioning material is provided on the periphery of a current collector such that a sealing portion of an exterior body (film) is located outside the cushioning material.

TECHNICAL FIELD

One embodiment of the present invention relates to an object, a method,or a manufacturing method. The present invention relates to a process, amachine, manufacture, or a composition of matter. One embodiment of thepresent invention relates to a semiconductor device, a display device, alight-emitting device, a power storage device, a lighting device, anelectronic device, or a manufacturing method thereof. In particular, oneembodiment of the present invention relates to an electronic device andits operating system.

Note that electronic devices in this specification mean all devicesincluding secondary batteries, and electro-optical devices includingsecondary batteries, information terminal devices including secondarybatteries, and the like are all electronic devices.

BACKGROUND ART

Portable electronic devices and wearable electronic devices are underactive development. For example, a thin portable electronic book isdisclosed in Patent Document 1.

Portable electronic devices and wearable electronic devices operateusing batteries as power sources. Portable electronic devices need towithstand extended use and thus can incorporate high-capacity secondarybatteries. In that case, there occurs a problem that the size and weightof the electronic devices are large. In view of this problem, a smalland thin high-capacity secondary battery that can be incorporated in aportable electronic device is under development.

Metal cans are used as exterior bodies of secondary batteries, andelectrolytes and the like are contained in the metal cans.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    S63-15796

DISCLOSURE OF INVENTION

A metal can used as an exterior body has a problem of increasing theweight of a secondary battery. Moreover, it is difficult to manufacturea thin metal can by molding and fabricate a secondary battery using athin metal can, in order to obtain a thin secondary battery.

The use of a film (also referred to as a laminate film) including astack of metal foil (e.g., aluminum foil or stainless steel foil) and aresin (heat-seal resin) as an exterior body allows fabrication of asecondary battery that is thinner and more lightweight than a secondarybattery using a metal can. An object of one embodiment of the presentinvention is to provide a novel power storage device, a novel secondarybattery, or the like. Note that the descriptions of these objects do notdisturb the existence of other objects. In one embodiment of the presentinvention, there is no need to achieve all the objects. Other objectswill be apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

In the case where a film, which has lower strength than a metal can, isused as an exterior body of a secondary battery, a current collectorprovided in a region surrounded by the exterior body, an active materiallayer provided on a surface of the current collector, or the like mightbe damaged when force is externally applied to the secondary battery.The current collector includes a projection (also referred to as anelectrode tab portion) for connection to a lead electrode. When thesecondary battery is bent by external force, part of the currentcollector suffers damage such as a crack in the vicinity of theprojection (electrode tab portion), leading to breakage of the secondarybattery.

A cushioning material is provided in a region surrounded by an exteriorbody of a secondary battery. Specifically, a cushioning material isprovided on the periphery of a current collector such that a sealingportion of an exterior body (film) is located outside the cushioningmaterial. To increase capacity, a plurality of units each including atleast a positive electrode current collector, a separator, and anegative electrode current collector are stacked in a region surroundedby an exterior body. Note that a cushioning material is provided betweenthe exterior body and the outermost current collector. The cushioningmaterial has a thickness larger than that of the separator and that ofthe current collector. Furthermore, the cushioning material may beformed using the same material as that for the separator so as to have arolled-sheet shape.

Examples of the shape of the cushioning material include a flat plateshape, a bar-like shape, a spherical shape (e.g., the shape of a plasticbead or a glass bead), and a rectangular solid shape. For example, asheet-like plastic film with slits may be employed. Furthermore, aplurality of cushioning materials may be provided in a region surroundedby an exterior body. The materials may have different sizes and shapes.For example, an aggregate including bound multiple fiber threads (glassfiber) may be used as the cushioning material. Alternatively, anaggregate (weave) including organic resin threads woven like a cloth maybe used. Furthermore, a sheet-like material that is rolled or folded maybe used. Specifically, the cushioning material (plastic film) with athin plate-like shape the area of which is larger than that of thecurrent collector is provided in the region surrounded by the exteriorbody, so as to overlap with the current collector. One embodimentdisclosed in this specification is a secondary battery including a film,and a first current collector, an active material layer, a secondcurrent collector, and a cushioning material in a region surrounded bythe film. The cushioning material is a plastic film whose area is largerthan the area of a region where the first current collector and thesecond current collector overlap with each other.

A frame-like cushioning material is provided in a region surrounded byan exterior body such that a planar shape of a combination of a currentcollector and the cushioning material that fits the outer shape of thecurrent collector is almost a rectangle. In this case, even when thecurrent collector and the cushioning material are not in contact witheach other in a region surrounded by the exterior body, sealing theouter edge of the exterior body by thermocompression bonding makes theboundary between the current collector and the cushioning materialunclear in appearance. In the appearance of a secondary batterydisclosed in this specification, the outer edge is thin because ofthermocompression bonding. A center portion where a stack (e.g., a firstcurrent collector, a separator, and a second current collector) existhas a larger thickness than the outer edge. A cushioning material isprovided between the center portion and the outer edge, and a portionbetween the center portion and the outer edge is thicker than the outeredge. Furthermore, a first lead electrode that is electrically connectedto the first current collector in the stack projects and is exposed, anda second lead electrode that is electrically connected to the secondcurrent collector projects and is exposed.

A material of the cushioning material is preferably an insulator (e.g.,plastic, rubber (natural rubber or synthetic rubber), glass, nonwovenfabric, or paper). In particular, a material of the cushioning materialis preferably an elastic material (e.g., synthetic rubber such assilicone rubber, fluorine rubber, chloroprene rubber, nitrile-butadienerubber, ethylene-propylene rubber, or styrene-butadiene rubber).Specifically, a material having a larger elastic modulus than theseparator is used as a material of the cushioning material.Alternatively, a porous material including air bubbles (e.g., sheet-likestyrofoam or sponge rubber formed using any of the synthetic rubbermaterials listed above) may be used as a material of the cushioningmaterial. A gelled material may be used as a material of the cushioningmaterial.

Alternatively, a conductive material that has an insulating surface canbe used as a material of the cushioning material. Examples of a materialof the cushioning material include carbon fiber whose surface is coatedwith an organic resin; metal foil (e.g., aluminum foil, copper foil, orstainless steel foil) having a surface over which an inorganicinsulating film such as a silicon oxide film is formed; and metal foilwhose surface is coated with an organic resin.

The cushioning material provided in the region surrounded by theexterior body in a secondary battery allows the current collector andthe like to be stably positioned. When the secondary battery is bent tohave a desired form, the cushioning material can also be bent so thatthe secondary battery can have the desired form, contributing tomaintaining of the bent form of the secondary battery. Furthermore, arestricting function of preventing the secondary battery from being bentmore than necessary may be provided. The cushioning material can alsoserve as a framework of the secondary battery.

The cushioning material is not necessarily provided in the regionsurrounded by the exterior body, and may be provided so as to be partlyexposed. In that case, the cushioning material itself serves as part ofthe exterior body, that is, a sealing material.

Depending on the mode of an electronic device, a secondary batteryprovided in the electronic device is bent, and it is preferred that acushioning material be also bendable. Thus, a material of the cushioningmaterial is preferably flexible. Furthermore, even when the volume of anelectrolytic solution in the secondary battery is reduced because ofdeterioration over time, generation of wrinkles and a change inappearance of the secondary battery by bending can be suppressed.Moreover, the cushioning material, which is located in a regionsurrounded by an exterior body, relieves concentration of an impulsethat is applied to the secondary battery, protecting the secondarybattery from being locally bent and damaged.

Another embodiment of the present invention is a power storage unitincluding a plurality of first current collectors for positiveelectrodes and second current collectors for negative electrodes in aregion surrounded by an exterior body.

The power storage unit of one embodiment of the present invention canchange its form with a curvature radius of greater than or equal to 10mm, preferably greater than or equal to 30 mm. One or two films are usedas the exterior body of the power storage unit. In the case where thepower storage unit has a layered structure, the power storage unit has across section sandwiched by two curves of the film(s) when bent.

Description is given of the curvature radius of a surface with referenceto FIGS. 7A to 7C. In FIG. 7A, on a plane 1701 along which a curvedsurface 1700 is cut, part of a curve 1702 forming the curved surface1700 is approximate to an arc of a circle, and the radius of the circleis referred to as a radius 1703 of curvature and the center of thecircle is referred to as a center 1704 of curvature. FIG. 7B is a topview of the curved surface 1700. FIG. 7C is a cross-sectional view ofthe curved surface 1700 taken along the plane 1701. When a curvedsurface is cut along a plane, the radius of curvature of a curve in across section differs depending on the angle between the curved surfaceand the plane or on the cut position, and the smallest radius ofcurvature is defined as the radius of curvature of a surface in thisspecification and the like.

In the case of bending a power storage unit in which a component 1805including electrodes and an electrolytic solution is sandwiched betweentwo films as exterior bodies, a radius 1802 of curvature of a film 1801close to a center 1800 of curvature of the power storage unit is smallerthan a radius 1804 of curvature of a film 1803 far from the center 1800of curvature (FIG. 8A). When the power storage unit is curved and has anarc-shaped cross section, compressive stress is applied to a surface ofthe film on the side closer to the center 1800 of curvature and tensilestress is applied to a surface of the film on the side farther from thecenter 1800 of curvature (FIG. 8B). However, by forming a patternincluding projections or depressions on surfaces of the exterior bodies,the influence of a strain can be reduced to be acceptable even whencompressive stress and tensile stress are applied. For this reason, thepower storage unit can change its form such that the exterior body onthe side closer to the center of curvature has a curvature radiusgreater than or equal to 10 mm, preferably greater than or equal to 30mm.

Note that the cross-sectional shape of the power storage unit is notlimited to a simple arc shape, and the cross section can be partlyarc-shaped; for example, a shape illustrated in FIG. 8C, a wavy shapeillustrated in FIG. 8D, or an S shape can be used. When the curvedsurface of the power storage unit has a shape with a plurality ofcenters of curvature, the power storage unit can change its form suchthat a curved surface with the smallest radius of curvature among radiiof curvature with respect to the plurality of centers of curvature,which is a surface of the exterior body on the side closer to the centerof curvature, has a curvature radius greater than or equal to 10 mm,preferably greater than or equal to 30 mm.

One embodiment of the present invention can be used for various powerstorage devices. Examples of such power storage devices include abattery, a primary battery, a secondary battery, a lithium-ion secondarybattery (including a lithium-ion polymer secondary battery), and alithium air battery. In addition, a capacitor is given as anotherexample of the power storage devices. For example, with a combination ofthe negative electrode of one embodiment of the present invention and anelectric double layer positive electrode, a capacitor such as alithium-ion capacitor can be fabricated.

The degree of change in form of a secondary battery due to externalforce, that is, a change in part of an internal structure of thesecondary battery due to external force can be adjusted by a material orthe position of a cushioning material. For example, the cushioningmaterial can suppress sharp bending so that the internal structure ofthe secondary battery is not broken. Thus, the cushioning material canprotect the internal structure from being damaged by external bendingforce. Furthermore, a novel power storage device, a novel secondarybattery, or the like can be provided. Note that the description of theseeffects does not disturb the existence of other effects. One embodimentof the present invention does not necessarily have all the effectslisted above. Other effects will be apparent from and can be derivedfrom the description of the specification, the drawings, the claims, andthe like.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are a schematic external view and a cross-sectional viewillustrating an embodiment of the present invention;

FIGS. 2A to 2C are perspective views illustrating structural examples ofa region surrounded by an external body of embodiments of the presentinvention;

FIGS. 3A and 3B are schematic external views illustrating a structuralexample of one embodiment of the present invention;

FIGS. 4A to 4E are schematic external views illustrating a structuralexample of one embodiment of the present invention and an X-rayphotograph showing one embodiment of the present invention;

FIGS. 5A to 5H are external perspective views illustrating electronicdevices of embodiments of the present invention;

FIGS. 6A to 6D illustrate electronic devices;

FIGS. 7A to 7C illustrate a radius of curvature of a surface; and

FIGS. 8A to 8D illustrate cross sections of a power storage unit.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings. However, the present invention is notlimited to the descriptions below, and it is easily understood by thoseskilled in the art that modes and details disclosed herein can bemodified in various ways. Further, the present invention is notconstrued as being limited to the descriptions of the embodiments.

Note that in each drawing referred to in this specification, the size orthe layer thickness of each component is exaggerated or a region of eachcomponent is omitted for clarity of the invention in some cases.Therefore, embodiments of the present invention are not limited to sucha scale.

Note that ordinal numbers such as “first” and “second” in thisspecification and the like are used in order to avoid confusion amongcomponents and do not denote the priority or the order such as the orderof steps or the stacking order. A term without an ordinal number in thisspecification and the like is provided with an ordinal number in a claimin some cases in order to avoid confusion among components.

Embodiment 1

FIG. 1A illustrates an example of a schematic view of a power storageunit. FIG. 2A illustrates an example of an internal structure surroundedby an exterior body of the power storage unit.

A power storage unit 100 of one embodiment of the present inventionincludes at least a positive electrode 101, a separator 103, a negativeelectrode 102, a cushioning material 110, and an electrolytic solutionin an exterior body 107. The power storage unit can have any of avariety of structures, and a film is used for formation of the exteriorbody 107 in this embodiment.

A film used to form the exterior body 107 is a single-layer filmselected from a metal film (a film of a metal in the form of foil, suchas aluminum, stainless steel, nickel steel, gold, silver, copper,titanium, nichrome, iron, tin, tantalum, niobium, molybdenum, zirconium,or zinc or an alloy thereof), a plastic film made of an organicmaterial, a hybrid material film containing an organic material (e.g.,an organic resin or fiber) and an inorganic material (e.g., ceramic),and a carbon-containing film (e.g., a carbon film or a graphite film);or a layered film including two or more of the above films.

In this embodiment, a sheet-like plastic film having a larger area thanthe positive electrode 101 is used as the cushioning material 110. Inthis embodiment, a plastic film having a larger thickness than theseparator 103 is used as the cushioning material 110. The cushioningmaterial 110 may be provided with slits. The shape of the cushioningmaterial 110 is not limited to a rectangle and may be a shape with fourround angles. If the shape of the cushioning material 110 has an acuteangle, when the power storage unit is bent, the angle might damage thefilm serving as the exterior body. Thus, angles of the cushioningmaterial 110 are chamfered, so that the power storage unit can have highreliability. An insulating material is used as a material of thecushioning material 110; for example, PP, PE, polyester such as PET orPBT, polyamide such as nylon 6 or nylon 66, an inorganic depositionfilm, or paper is used.

The cushioning material 110 that is provided in a region surrounded bythe exterior body of the power storage unit allows the current collectorand the like to be stably positioned. When the power storage unit isbent to have a desired form, the cushioning material can also be bent sothat the power storage unit can have the desired form, contributing tomaintaining of the bent form of the power storage unit. Furthermore, arestricting function of preventing the power storage unit from beingbent more than necessary may be provided. The cushioning material canalso serve as a framework of the power storage unit. By providing thecushioning material 110 in the region surrounded by the exterior body ofthe power storage unit, the influence of a strain caused by externallyapplying force to the power storage unit can be reduced to beacceptable. Thus, the power storage unit can have high reliability.

Furthermore, when the cushioning material 110 provided in the regionsurrounded by the exterior body of the power storage unit is a plasticfilm with a smooth surface, the cushioning material 110 can be slid witha current corrector in contact with the surface of the cushioningmaterial 110 and the exterior body in contact with the surface of thecushioning material 110. Thus, the power storage unit can haveresistance to repeated bending.

Note that the positive electrode 101 refers to the one including acurrent collector (e.g., aluminum) with one surface or opposite surfaceseach provided with a positive electrode active material layer and thelike. The negative electrode 102 refers to the one including a currentcollector (e.g., copper) with one surface or opposite surfaces eachprovided with a negative electrode active material layer and the like.The positive electrode 101 is electrically connected to a positiveelectrode lead 104. The negative electrode 102 is electrically connectedto a negative electrode lead 105. Each of the positive electrode lead104 and the negative electrode lead 105 is also referred to as a leadelectrode or a lead terminal. Parts of the positive electrode lead 104and the negative electrode lead 105 are positioned outside the exteriorbody. The power storage unit 100 is charged and discharged through thepositive electrode lead 104 and the negative electrode lead 105.

Here, a current flow in charging a secondary battery will be describedwith reference to FIG. 1B. When a secondary battery using lithium isregarded as a closed circuit, lithium ions transfer and a current flowsin the same direction. Note that in the secondary battery using lithium,an anode and a cathode change places in charge and discharge, and anoxidation reaction and a reduction reaction occur on the correspondingsides; hence, an electrode with a high redox potential is called apositive electrode and an electrode with a low redox potential is calleda negative electrode. For this reason, in this specification, thepositive electrode is referred to as a “positive electrode” and thenegative electrode is referred to as a “negative electrode” in all thecases where charge is performed, discharge is performed, a reverse pulsecurrent is supplied, and a charging current is supplied. The use of theterms “anode” and “cathode” related to an oxidation reaction and areduction reaction might cause confusion because the anode and thecathode change places at the time of charging and discharging. Thus, theterms “anode” and “cathode” are not used in this specification. If theterm “anode” or “cathode” is used, it should be mentioned that the anodeor the cathode is which of the one at the time of charging or the one atthe time of discharging and corresponds to which of a positive electrodeor a negative electrode.

Two terminals in FIG. 1B are connected to a charger, and the powerstorage unit 100 is charged. As the charge of the power storage unit 100proceeds, a potential difference between electrodes increases. Thepositive direction in FIG. 1B is the direction in which a current flowsfrom one terminal (positive electrode lead 104) outside the powerstorage unit 100 to a positive electrode current collector (positiveelectrode 101), and flows from the positive electrode 101 to thenegative electrode 102 and from the negative electrode to the otherterminal (negative electrode lead 105) outside the power storage unit100 in the power storage unit 100. In other words, a current flows inthe direction of a flow of a charging current.

In this embodiment, the example in which one pair of the positiveelectrode 101 and the negative electrode 102 is provided in the exteriorbody is described for simplicity. However, a plurality of pairs of thepositive electrode 101 and the negative electrode 102 may be provided inthe exterior body 107 to increase the capacity of the power storageunit.

FIG. 2A illustrates an example of arrangement in the exterior body 107;the cushioning material 110, the positive electrode 101, the separator103, and the negative electrode 102 are arranged as in FIG. 2A. Notethat the exterior body 107, the positive electrode lead 104, and thenegative electrode lead 105 are not illustrated in FIG. 2A forsimplicity.

Examples of materials for forming the separator 103 include porousinsulators such as cellulose, polypropylene (PP), polyethylene (PE),polybutene, nylon, polyester, polysulfone, polyacrylonitrile,polyvinylidene fluoride, and tetrafluoroethylene. Alternatively,nonwoven fabric of a glass fiber or the like, or a diaphragm in which aglass fiber and a polymer fiber are mixed can be used.

In this embodiment, the structure of the power storage unit is asfollows, for example: the thickness of the separator 103 isapproximately 15 μm to 30 μm; the thickness of a current corrector inthe positive electrode 101 is approximately 10 μm to 40 μm; thethickness of a positive electrode active material layer is approximately50 μm to 100 μm; the thickness of a negative electrode active materiallayer is approximately 50 μm to 100 μm; and the thickness of a currentcorrector in the negative electrode 102 is approximately 5 μm to 40 μm.

Although a sheet-like separator may be used as the separator 103 in FIG.2A, a bag-like one may alternatively be used. Furthermore, one separatormay be bent and provided in the exterior body 107 such that the positiveelectrode (or the negative electrode) is located between facing surfacesof the bent separator.

Furthermore, the position of the cushioning material 110 is not limitedto that in FIG. 2A. For example, the cushioning material 110 may beprovided on the side in contact with the negative electrode 102 asillustrated in FIG. 2B.

Furthermore, the number of the cushioning material 110 is notnecessarily one and may be more than one. For example, the positiveelectrode 101, the separator 103, and the negative electrode 102 may beprovided between a first cushioning material 110 a and a secondcushioning material 110 b as illustrated in FIG. 2C.

Examples of positive electrode active materials that can be used for thepositive electrode active material layer in the power storage unit 100include a composite oxide with an olivine structure, a composite oxidewith a layered rock-salt structure, and a composite oxide with a spinelstructure. Specifically, a compound such as LiFeO₂, LiCoO₂, LiNiO₂,LiMn₂O₄, V₂O₅, Cr₂O₅, or MnO₂ can be used.

Alternatively, a complex material (LiMPO₄ (general formula) (M is one ormore of Fe(II), Mn(II), Co(II), and Ni(II))) can be used. Typicalexamples of the general formula LiMPO₄ which can be used as a materialare lithium compounds such as LiFePO₄, LiNiPO₄, LiCoPO₄, LiMnPO₄,LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄,LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≦1, 0<a<1, and 0<b<1),LiFe_(c)Ni_(d)Co_(e)PO₄, LiFe_(c)Ni_(d)Mn_(e)PO₄,LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≦1, 0<c<1, 0<d<1, and 0<e<1), andLiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≦1, 0<f<1, 0<g<1, 0<h<1, and0<i<1).

Alternatively, a complex material such as Li_((2-j))MSiO₄ (generalformula) (M is one or more of Fe(II), Mn(II), Co(II), and Ni(II); 0≦j≦2)may be used. Typical examples of the general formula Li_((2-j))MSiO₄which can be used as a material are lithium compounds such asLi_((2-j))FeSiO₄, Li_((2-j))NiSiO₄, Li_((2-j))COSiO₄, Li_((2-j))MnSiO₄,Li_((2-j))Fe_(k)Ni/SiO₄, Li_((2-j))Fe_(k)Co_(l)SiO₄,Li_((2-j))Fe_(k)Mn_(l)SiO₄, Li_((2-j))Ni_(k)Co_(l)SiO₄,Li_((2-j))Ni_(k)Mn_(l)SiO₄ (k+l≦1, 0<k<1, and 0<l<1),Li_((2-j))Fe_(m)Ni_(n)Co_(q)SiO₄, Li_((2-j))Fe_(m)Ni_(n)Mn_(q)SiO₄,Li_((2-j))Ni_(m)Co_(n)Mn_(q)SiO₄ (m+n+q≦1, 0<m<1, 0<n<1, and 0<q<1), andLi_((2-j))Fe_(r)Ni_(s)Co_(t)Mn_(u)SiO₄ (r+s+t+u≦1, 0<r<1, 0<s<1, 0<t<1,and 0<u<1).

Still alternatively, a nasicon compound expressed by A_(x)M₂(XO₄)₃(general formula) (A=Li, Na, or Mg, M=Fe, Mn, Ti, V, Nb, or Al, X═S, P,Mo, W, As, or Si) can be used for the positive electrode activematerial. Examples of the nasicon compound are Fe₂(MnO₄)₃, Fe₂(SO₄)₃,and Li₃Fe₂(PO₄)₃. Further alternatively, a compound expressed byLi₂MPO₄F, Li₂MP₂O₇, or Li₅MO₄ (general formula) (M=Fe or Mn), aperovskite fluoride such as NaFeF₃ and FeF₃, a metal chalcogenide (asulfide, a selenide, or a telluride) such as TiS₂ and MoS₂, an oxidewith an inverse spinel structure such as LiMVO₄, a vanadium oxide (V₂O₅,V₆O₁₃, LiV₃O₈, or the like), a manganese oxide, an organic sulfurcompound, or the like can be used as the positive electrode activematerial.

In the case where carrier ions are alkali metal ions other than lithiumions, or alkaline-earth metal ions, a material containing an alkalimetal (e.g., sodium and potassium) or an alkaline-earth metal (e.g.,calcium, strontium, barium, beryllium, and magnesium) instead of lithiummay be used as the positive electrode active material.

As the separator 103, an insulator such as cellulose (paper),polypropylene with pores, or polyethylene with pores can be used.

As an electrolyte of the electrolytic solution, a material that hascarrier ion mobility and contains lithium ions as carrier ions is used.Typical examples of the electrolyte are lithium salts such as LiPF₆,LiClO₄, LiAsF₆, LiBF₄, LiCF₃SO₃, Li(CF₃SO₂)₂N, and Li(C₂F₅SO₂)₂N. One ofthese electrolytes may be used alone, or two or more of them may be usedin an appropriate combination and in an appropriate ratio.

As a solvent of the electrolytic solution, a material with carrier ionmobility is used. As the solvent of the electrolytic solution, anaprotic organic solvent is preferably used. Typical examples of aproticorganic solvents include ethylene carbonate (EC), propylene carbonate,dimethyl carbonate, diethyl carbonate (DEC), γ-butyrolactone,acetonitrile, dimethoxyethane, tetrahydrofuran, and the like, and one ormore of these materials can be used. When a gelled high-molecularmaterial is used as the solvent of the electrolytic solution, safetyagainst liquid leakage and the like is improved. Furthermore, thestorage battery can be thinner and more lightweight. Typical examples ofgelled high-molecular materials include a silicone gel, an acrylic gel,an acrylonitrile gel, a polyethylene oxide-based gel, a polypropyleneoxide-based gel, a fluorine-based polymer gel, and the like.Alternatively, the use of one or more kinds of ionic liquids (roomtemperature molten salts) which have features of non-flammability andnon-volatility as a solvent of the electrolytic solution can prevent thestorage battery from exploding or catching fire even when the storagebattery internally shorts out or the internal temperature increasesowing to overcharging and others. An ionic liquid is a salt in theliquid state and has high ion mobility (conductivity). An ionic liquidcontains a cation and an anion. Examples of ionic liquids include anionic liquid containing an ethylmethylimidazolium (EMI) cation and anionic liquid containing an N-methyl-N-propylpiperidinium (PP₁₃) cation.

Instead of the electrolytic solution, a solid electrolyte including aninorganic material such as a sulfide-based inorganic material or anoxide-based inorganic material, or a solid electrolyte including amacromolecular material such as a polyethylene oxide (PEO)-basedmacromolecular material may alternatively be used. When the solidelectrolyte is used, a separator and a spacer are not necessary.Furthermore, the battery can be entirely solidified; therefore, there isno possibility of liquid leakage and thus the safety of the battery isdramatically increased.

A material with which lithium can be dissolved and precipitated or amaterial into and from which lithium ions can be inserted and extractedcan be used for a negative electrode active material used in thenegative electrode active material layer in the power storage unit 100;for example, a lithium metal, a carbon-based material, an alloy-basedmaterial, or the like can be used.

The lithium metal is preferable because of its low redox potential(3.045 V lower than that of a standard hydrogen electrode) and highspecific capacity per unit weight and per unit volume (3860 mAh/g and2062 mAh/cm³).

Examples of the carbon-based material include graphite, graphitizingcarbon (soft carbon), non-graphitizing carbon (hard carbon), a carbonnanotube, graphene, carbon black, and the like.

Examples of the graphite include artificial graphite such as meso-carbonmicrobeads (MCMB), coke-based artificial graphite, or pitch-basedartificial graphite and natural graphite such as spherical naturalgraphite.

Graphite has a low potential substantially equal to that of a lithiummetal (0.1 V to 0.3 V vs. Li/Li⁺) when lithium ions are intercalatedinto the graphite (while a lithium-graphite intercalation compound isformed). For this reason, a lithium-ion secondary battery can have ahigh operating voltage. In addition, graphite is preferable because ofits advantages such as relatively high capacity per unit volume, smallvolume expansion, low cost, and safety greater than that of a lithiummetal.

For the negative electrode active material, an alloy-based material oran oxide which enables charge-discharge reactions by an alloyingreaction and a dealloying reaction with lithium can be used. In the casewhere carrier ions are lithium ions, a material containing at least oneof Al, Si, Ge, Sn, Pb, Sb, Bi, Ag, Au, Zn, Cd, In, Ga, and the like canbe used as the alloy-based material, for example. Such elements havehigher capacity than carbon. In particular, silicon has a significantlyhigh theoretical capacity of 4200 mAh/g. For this reason, silicon ispreferably used as the negative electrode active material. Examples ofthe alloy-based material that uses such an element include Mg₂Si, Mg₂Ge,Mg₂Sn, SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb,CeSb₃, LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, SbSn, and the like. Note that SiOrefers to the powder of a silicon oxide including a silicon-rich portionand can also be referred to as SiO_(y) (2>y>0). Examples of SiO includea material containing one or more of Si₂O₃, Si₃O₄, and Si₂O and amixture of Si powder and silicon dioxide (SiO₂). Furthermore, SiO maycontain another element (e.g., carbon, nitrogen, iron, aluminum, copper,titanium, calcium, and manganese). In other words, SiO refers to acolored material containing two or more of single crystal silicon,amorphous silicon, polycrystal silicon, Si₂O₃, Si₃O₄, Si₂O, and SiO₂.Thus, SiO can be distinguished from SiO_(x) (x is 2 or more), which isclear and colorless or white. Note that in the case where a secondarybattery is fabricated using SiO as a material thereof and the SiO isoxidized because of repeated charge and discharge cycles, SiO is changedinto SiO₂ in some cases.

Alternatively, for the negative electrode active material, SiO, SnO,SnO₂, or an oxide such as titanium dioxide (TiO₂), lithium titaniumoxide (Li₄Ti₅O₁₂), lithium-graphite intercalation compound (Li_(x)C₆),niobium pentoxide (Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide(MoO₂) can be used.

Still alternatively, for the negative electrode active materials,Li_(3-x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm³).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are contained in the negative electrodeactive materials and thus the negative electrode active materials can beused in combination with a material for a positive electrode activematerial which does not contain lithium ions, such as V₂O₅ or Cr₃O₈. Inthe case of using a material containing lithium ions as a positiveelectrode active material, the nitride containing lithium and atransition metal can be used for the negative electrode active materialby extracting the lithium ions contained in the positive electrodeactive material in advance.

Alternatively, a material which causes a conversion reaction can be usedfor the negative electrode active materials; for example, a transitionmetal oxide which does not cause an alloy reaction with lithium, such ascobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may beused. Other examples of the material which causes a conversion reactioninclude oxides such as Fe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides suchas CoS_(0.89), NiS, and CuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄,phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ andBiF₃. Note that any of the fluorides can be used as a positive electrodeactive material because of its high potential.

The negative electrode active material layer may further include abinder for increasing adhesion of active materials, a conductiveadditive for increasing the conductivity of the negative electrodeactive material layer, and the like in addition to the above negativeelectrode active materials.

The cushioning material is not necessarily provided in the regionsurrounded by the exterior body, and may be provided so as to be partlyexposed. In bonding outer edges of the exterior body 107 bythermocompression bonding, thermocompression bonding may be performedfor sealing with a bonding region and part of the sheet-like cushioningmaterial overlapping with each other. In that case, the sheet-likecushioning material is fixed in a portion in contact with the bondingregion.

Although an example of a small battery used in a portable informationterminal or the like is described in this embodiment, one embodiment ofthe present invention is not particularly limited to this example.Application to a large battery provided in a vehicle or the like is alsopossible.

Embodiment 2

In Embodiment 1, a sheet-like cushioning material is used. In thisembodiment, a cushioning material having a shape different from that inEmbodiment 1 is used and provided at a position different from that inEmbodiment 1.

FIG. 3A illustrates an example of a schematic view of a power storageunit. FIG. 3B illustrates an example of an internal structure surroundedby an exterior body of a power storage unit. Note that in FIGS. 3A and3B, the same reference numerals are used for the same parts as those inFIGS. 1A and 1B, and detailed description of the parts is omitted herefor simplicity.

A power storage unit 300 of one embodiment of the present inventionincludes at least the positive electrode 101, the separator 103, thenegative electrode 102, a first cushioning material 310 a, a secondcushioning material 310 b, and an electrolytic solution in the exteriorbody 107.

In this embodiment, bar-like elastic bodies (elastic materials) that areeach thicker than the separator 103 are used as the first cushioningmaterial 310 a and the second cushioning material 310 b.

As illustrated in FIGS. 3A and 3B, the positive electrode 101, theseparator 103, and the negative electrode 102 are positioned between thefirst cushioning material 310 a and the second cushioning material 310b.

Facing outer edges of the exterior body 107 are bonded bythermocompression bonding. A surface of a film used as the exterior body107 is provided with a polypropylene layer, and only the portionsubjected to thermocompression bonding is a bonding region.

In this embodiment, a bonding region 311 is in contact with the firstcushioning material 310 a, and the first cushioning material 310 a isexposed at a side surface of the power storage unit 300. In the casewhere the cushioning material is not exposed and provided inside thepower storage unit, a wedge shape is employed as a cross-sectional shapeof the cushioning material to make a step in the vicinity of a sealingportion gentle. In addition, the bonding region is in contact with thesecond cushioning material 310 b, and the second cushioning material 310b is exposed at the side surface of the power storage unit 300. Thefirst cushioning material 310 a and the second cushioning material 310 balso serve as sealing materials.

As a material for the first cushioning material 310 a and the secondcushioning material 310 b, a one having a higher elasticity than that ofthe separator is used. For example, rubber (e.g., natural rubber orsynthetic rubber) is used. Note that a material that is less likely tochemically react by contact with an electrolytic solution is preferablyselected. In this embodiment, silicone rubber, which is less likely tochemically react by contact with an electrolytic solution, is used.Furthermore, a portion of the material used for the first cushioningmaterial 310 a and the second cushioning material 310 b that is incontact with an electrolytic solution is preferably coated with amaterial with a high solvent resistance to an electrolytic solution andsubjected to surface treatment.

As a material for the first cushioning material 310 a and the secondcushioning material 310 b, a one that can be bonded by thermocompressionbonding is used.

The provided first cushioning material 310 a and second cushioningmaterial 310 b can suppress generation of wrinkles on the film servingas the exterior body in the outer edge of the power storage unit 300even when the power storage unit 300 is bent.

When multiple combinations of the positive electrode 101, the separator103, and the negative electrode 102 that are stacked is packed in theexterior body to increase the capacity of the power storage unit 300,the total thickness increases, increasing the difference between thetotal thickness and the thickness of the outer edge. Accordingly, a stepof the exterior body is formed. The first cushioning material 310 a andthe second cushioning material 310 b are preferably provided to reducethe step.

Furthermore, the lengths of the first cushioning material 310 a and thesecond cushioning material 310 b may be increased to provide a space sothat the current collector and the like can be slid when the powerstorage unit 300 is bent.

Although the example where two elastic bodies of the first cushioningmaterial 310 a and the second cushioning material 310 b are used isdescribed, one embodiment of the present invention is not particularlylimited to this example. One U-shaped elastic material or frame-likeelastic material may be used as a cushioning material.

This embodiment can be combined with Embodiment 1. For example, thepositive electrode 101, the separator 103, and the negative electrode102 that are positioned between the first cushioning material 310 a andthe second cushioning material 310 b and a sheet-like plastic film as athird cushioning material may be packed in the power storage unit.

Embodiment 3

In this embodiment, unevenness is formed on a film serving as anexterior body by pressing, e.g., embossing, and a sheet-like plasticfilm is used as the cushioning material 110 in a region surrounded bythe exterior body.

In this embodiment, an example of fabricating a lithium-ion secondarybattery with the use of a film whose surface is embossed with a patternwill be described with reference to FIGS. 4A to 4E. Note that in FIGS.4A to 4E, the same reference numerals are used for the same parts asthose in FIGS. 1A and 1B, and detailed description of the parts isomitted here for simplicity.

First, a sheet made of a flexible material is prepared. As the sheet, astacked body, a metal film provided with an adhesive layer (alsoreferred to as a heat-seal layer) or sandwiched between adhesive layersis used. As the adhesive layer, a heat-seal resin film containing, e.g.,polypropylene or polyethylene is used. In this embodiment, a metalsheet, specifically, aluminum foil whose top surface is provided with anylon resin and whose bottom surface is provided with a stack includingan acid-proof polypropylene film and a polypropylene film is used as thesheet. This sheet is cut to obtain a film.

Then, the film is embossed to form unevenness so that the pattern can bevisually recognized. Although an example where the sheet is cut and thenembossing is performed is described here, the order is not particularlylimited; embossing may be performed before cutting the sheet and thenthe sheet may be cut. Alternatively, the sheet may be cut afterthermocompression bonding is performed with the sheet bent.

Note that embossing refers to processing for forming unevenness on asurface of a film by bringing an embossing roll whose surface hasunevenness into contact with the film with pressure. The embossing rollis a roll whose surface is patterned.

The embossing roll is not necessarily used, and an embossing plate maybe used. Furthermore, embossing is not necessarily employed, and anymethod that allows formation of a relief on part of the film isemployed.

In this embodiment, both surfaces of a film 411 are provided withunevenness to have patterns, and the film 411 is folded in half so thattwo end portions each including two of the four corners overlap witheach other, and is sealed on three sides with an adhesive layer.

Then, the film 411 is folded in half, whereby a state illustrated inFIG. 4A is produced.

The positive electrode 101, the separator 103, and the negativeelectrode 102 that are stacked and included in a secondary battery asillustrated in FIG. 4B and the cushioning material 110 are prepared.Current collectors used in the positive electrode 101 and the negativeelectrode 102 can each be formed using a highly conductive material thatis not alloyed with a carrier ion of, for example, lithium, such as ametal typified by stainless steel, gold, platinum, zinc, iron, nickel,copper, aluminum, titanium, and tantalum or an alloy thereof.Alternatively, an aluminum alloy to which an element which improves heatresistance, such as silicon, titanium, neodymium, scandium, andmolybdenum, is added can be used. Still alternatively, a metal elementwhich forms silicide by reacting with silicon can be used. Examples ofthe metal element which forms silicide by reacting with silicon includezirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium,molybdenum, tungsten, cobalt, nickel, and the like. The currentcollectors can each have a foil-like shape, a plate-like shape(sheet-like shape), a net-like shape, a cylindrical shape, a coil shape,a punching-metal shape, an expanded-metal shape, or the like asappropriate. The current collectors each preferably have a thickness of5 μm to 40 μm inclusive. Note that the example where one combination ofthe positive electrode 101, the separator 103, and the negativeelectrode 102 that are stacked is packed in an exterior body isillustrated here for simplicity. To increase the capacity of a secondarybattery, a plurality of combinations may be stacked and packed in anexterior body. In this embodiment, 12 combinations are stacked andpacked in an exterior body.

In addition, two lead electrodes with the sealing layers 415 illustratedin FIG. 4C are prepared. The lead electrodes are each also referred toas a lead terminal and provided in order to lead a positive electrode ora negative electrode of a secondary battery to the outside of anexterior film. Aluminum and nickel-plated copper are used as thepositive electrode lead 104 and the negative electrode lead 105,respectively.

Then, the positive electrode lead 104 is electrically connected to aprotruding portion of the positive electrode 101 by ultrasonic weldingor the like. The negative electrode lead 105 is electrically connectedto a protruding portion of the negative electrode 102 by ultrasonicwelding or the like.

Then, two sides of the film 411 are sealed by thermocompression bonding,and one side is left open for introduction of an electrolytic solution.In thermocompression bonding, the sealing layers 415 provided on thelead electrodes are also melted, thereby fixing the lead electrodes andthe film 411 to each other. After that, in a reduced-pressure atmosphereor an inert atmosphere, a desired amount of electrolytic solution isintroduced to the inside of the film 411 in the form of a bag. Lastly,the outer edge of the film that has not been subjected tothermocompression bonding and is left open is sealed bythermocompression bonding.

In this manner, a power storage unit 400 illustrated in FIG. 4D can befabricated.

In the obtained power storage unit 400, the surface of the film 411serving as an exterior body has a pattern including unevenness. An edgeregion is a thermocompression-bonded region. A surface of thethermocompression-bonded region also has a pattern including unevenness.Although the unevenness in the thermocompression-bonded region issmaller than that in a center portion, it can relieve stress appliedwhen the secondary battery is bent. Such a structure as can relieve astrain caused by stress can prevent the secondary battery (e.g., anexterior body) from being damaged when changed in form by being bent,for example, achieving long-time reliability.

FIG. 4E is an X-ray photograph showing the case where a fabricatedsample having the above structure without an electrolytic solution thatis taken along A1-A2.

It is found from FIG. 4E that an exterior film is uneven and there is aspace between the lower side of the exterior film and a currentcollector. A plastic film is not observed on the X-ray, and thesheet-like plastic film (with a thickness of 300 μm) is provided in thisspace.

The sheet-like plastic film provided as a cushioning material canprevent unevenness of the exterior film and the current collector frombeing directly in contact with each other and thus damaging them whenthe power storage unit is bent.

This embodiment can be freely combined with Embodiment 1 or 2.

Embodiment 4

In this embodiment, examples of electronic devices each incorporatingthe power storage unit obtained using any one of Embodiments 1 to 3 willbe described.

Examples of electronic devices each using a power storage unit are asfollows: display devices (also referred to as televisions or televisionreceivers) such as head-mounted displays and goggle type displays,desktop personal computers, laptop personal computers, monitors forcomputers or the like, cameras such as digital cameras or digital videocameras, digital photo frames, electronic notebooks, e-book readers,electronic translators, toys, audio input devices such as microphones,electric shavers, electric toothbrushes, high-frequency heatingappliances such as microwave ovens, electric rice cookers, electricwashing machines, electric vacuum cleaners, water heaters, electricfans, hair dryers, air-conditioning systems such as humidifiers,dehumidifiers, and air conditioners, dishwashers, dish dryers, clothesdryers, futon dryers, electric refrigerators, electric freezers,electric refrigerator-freezers, freezers for preserving DNA,flashlights, electric power tools, alarm devices such as smokedetectors, gas alarm devices, and security alarm devices, industrialrobots, health equipment and medical equipment such as hearing aids,cardiac pacemakers, X-ray equipment, radiation counters, electricmassagers, and dialyzers, mobile phones (also referred to as mobilephone devices or cell phones), portable game machines, portableinformation terminals, lighting devices, headphone stereos, stereos,remote controls, clocks such as table clocks and wall clocks, cordlessphone handsets, transceivers, pedometers, calculators, portable orstationary music reproduction devices such as digital audio players, andlarge game machines such as pachinko machines.

The power storage unit fabricated according to any of Embodiments 1 to 3includes, as an exterior body, a thin film having flexibility and thuscan be bonded to a support structure body with a curved surface and canchange its form reflecting the curved surface of a region of the supportstructure body that has a large radius of curvature.

In addition, a flexible power storage unit can be incorporated along acurved inside/outside wall surface of a house or a building or a curvedinterior/exterior surface of an automobile.

FIG. 5A illustrates an example of a mobile phone. A mobile phone 7400includes a display portion 7402 incorporated in a housing 7401, anoperation button 7403, an external connection port 7404, a speaker 7405,a microphone 7406, and the like. Note that the mobile phone 7400includes a power storage unit 7407.

FIG. 5B illustrates the mobile phone 7400 that is bent. When the wholemobile phone 7400 is bent by external force, the power storage unit 7407included in the mobile phone 7400 is also bent. FIG. 5C illustrates thebent power storage unit 7407. The power storage unit 7407 is a laminatedstorage battery (also referred to as a layered battery or a film-coveredbattery). The power storage unit 7407 is fixed while being bent. Notethat the power storage unit 7407 includes a lead electrode 7408electrically connected to a current collector 7409. For example, acushioning material is provided in a region surrounded by a film servingas an exterior body of the power storage unit 7407, so that the powerstorage unit 7407 has high reliability even when bent. The mobile phone7400 may further be provided with a slot for insertion of a SIM card, aconnector portion for connecting a USB device such as a USB memory.

FIG. 5D illustrates an example of a mobile phone that can be bent. Whenbent to be put around a forearm, the mobile phone can be used as abangle-type mobile phone as in FIG. 5E. A mobile phone 7100 includes ahousing 7101, a display portion 7102, an operation button 7103, and apower storage unit 7104. FIG. 5F illustrates the power storage unit 7104that can be bent. When the mobile phone is worn on a user's arm whilethe power storage unit 7104 is bent, the housing changes its form andthe curvature of a part or the whole of the power storage unit 7104 ischanged. Specifically, a part or the whole of the housing or the mainsurface of the power storage unit 7104 is changed in the range of radiusof curvature from 10 mm to 150 mm. Note that the power storage unit 7104includes a lead electrode 7105 that is electrically connected to acurrent collector 7106. For example, a cushioning material is providedin a region surrounded by a film serving as an exterior body of thepower storage unit 7104, so that the power storage unit 7104 can retainhigh reliability even when the power storage unit 7104 is bent manytimes with different curvatures. As described above, the mobile phoneillustrated in FIG. 5D can be changed in form in more than one way, andit is desirable that at least the housing 7101, the display portion7102, and the power storage unit 7104 have flexibility in order tochange the form of the mobile phone.

The mobile phone 7100 may further be provided with a slot for insertionof a SIM card, a connector portion for connecting a USB device such as aUSB memory.

As for another usage example of the mobile phone, when a center portionof the mobile phone illustrated in FIG. 5D is bent, a form illustratedin FIG. 5G can be obtained. When a center portion of the mobile phone isfolded so that end portions of the mobile phone overlap with each otheras illustrated in FIG. 5H, the mobile phone can be reduced in size so asto be put in, for example, a pocket of clothes a user wears. In thecases of only changes in form illustrated in FIGS. 5D, 5G, and 5H, thepower storage unit 7104 is not being bent. When a mobile phone that isthin falls or has any other impact, the power storage unit 7104 providedtherein is also shocked. A cushioning material that is provided in aregion surrounded by a film serving as an exterior body of the powerstorage unit 7104 relieves such an impact, and the use of the cushioningmaterial enables fabrication of a durable secondary battery. Thus, theuse of the power storage unit 7104 provided with a cushioning materialin a region surrounded by a film serving as an exterior body allowsfabrication of a mobile phone that is highly reliable regardless ofwhether the mobile phone is bent or not.

FIG. 6A illustrates an example of a vacuum cleaner. By being providedwith a secondary battery, the vacuum cleaner can be cordless. To leave adust collecting space for storing vacuumed dust inside the vacuumcleaner, a space occupied by a power storage unit 7604 is preferably assmall as possible. For this reason, it is useful to provide the powerstorage unit 7604 that can be bent, between the outside surface and thedust collecting space.

The vacuum cleaner 7600 is provided with operation buttons 7603 and thepower storage unit 7604. FIG. 6B illustrates the power storage unit 7604that is capable of being bent. A cushioning material is provided in aregion surrounded by a film serving as an exterior body of the powerstorage unit 7604, so that the power storage unit 7604 has highreliability even when bent. The power storage unit 7604 includes a leadelectrode 7601 electrically connected to a negative electrode and a leadelectrode 7602 electrically connected to a positive electrode.

As another example of the power storage unit 7604 where two leadelectrodes are exposed from one short side of an exterior body, thepower storage unit 7605 that is capable of being bent is illustrated inFIG. 6C. Current collectors or lead electrodes of the power storage unit7605 are exposed from two short sides of an exterior body. Thecushioning material provided in the region surrounded by the filmserving as the exterior body of the power storage unit 7605 allows thepower storage unit 7605 to be bent, improving the reliability thereof.

FIG. 6D illustrates an example of the internal structure of the powerstorage unit 7605. As illustrated in FIG. 6D, the power storage unit7605 includes the positive electrode 101, the separator 103, and twonegative electrodes 102. The separator 103 is folded and the positiveelectrode 101 is provided inside the folded separator 103. In addition,the positive electrode 101 is sandwiched between positive electrodeactive material layers. Although FIG. 6D illustrates a combination ofone positive electrode and two negative electrodes, more combinations ofthe positive electrode and the negative electrodes may be used toincrease the capacity of the power storage unit 7605. The power storageunit 7605 also includes the cushioning material 110 in contact with oneof the negative electrodes 102. The cushioning material 110 is locatedin the region surrounded by the film in the power storage unit 7605 andhas a function of increasing the mechanical strength of the powerstorage unit 7605.

The thin power storage unit 7604 can be fabricated by the method forfabricating a laminated secondary battery that is described inEmbodiment 3.

The thin power storage unit 7604 has a laminated structure and is bentand fixed. The vacuum cleaner 7600 includes a display portion 7606 thatdisplays, for example, the remaining amount of power in the thin powerstorage unit 7604. A display area of the display portion 7606 is curvedto fit the shape of the outer surface of the vacuum cleaner. The vacuumcleaner includes a connection cord for being connected to a receptacle.When the thin power storage unit 7604 is charged to have sufficientpower, the connection cord can be removed from the receptacle to use thevacuum cleaner. The thin power storage unit 7604 may be chargedwirelessly without using the connection cord. The cushioning materialprovided in the region surrounded by the film serving as the exteriorbody of the power storage unit 7604 allows the power storage unit 7604to be shock-resistant and highly reliable.

The use of power storage units that can be bent in vehicles enablesproduction of next-generation clean energy vehicles such as hybridelectric vehicles (HEVs), electric vehicles (EVs), and plug-in hybridelectric vehicles (PHEVs). Moreover, power storage units that can bebent can also be used in moving objects such as agricultural machines,motorized bicycles including motor-assisted bicycles, motorcycles,electric wheelchairs, electric carts, boats or ships, submarines,aircrafts such as fixed-wing aircrafts and rotary-wing aircrafts,rockets, artificial satellites, space probes, planetary probes, andspacecrafts.

EXPLANATION OF REFERENCE

-   100: power storage unit, 101: positive electrode, 102: negative    electrode, 103: separator, 104: positive electrode lead, 105:    negative electrode lead, 107: exterior body, 110: cushioning    material, 110 a: cushioning material, 110 b: cushioning material,    300: power storage unit, 310 a: cushioning material, 310 b:    cushioning material, 311: bonding region, 400: power storage unit,    411: film, 415: sealing layer, 1700: curved surface, 1701: plane,    1702: curve, 1703: radius of curvature, 1704: center of curvature,    1800: center of curvature, 1801: film, 1802: radius of curvature,    1803: film, and 1804: radius of curvature

This application is based on Japanese Patent Application serial no.2014-102894 filed with Japan Patent Office on May 16, 2014, the entirecontents of which are hereby incorporated by reference.

1. A secondary battery comprising: a first current collector, an activematerial layer, a second current collector, a cushioning material, and afilm, wherein the film surrounds the first current collector, the activematerial layer, the second current collector, and the cushioningmaterial.
 2. The secondary battery according to claim 1, furthercomprising a separator between a surface of the first current collectorand a surface of the second current collector, wherein the cushioningmaterial has a larger thickness than the separator.
 3. The secondarybattery according to claim 1, wherein the cushioning material is anelastic body.
 4. The secondary battery according to claim 1, wherein thecushioning material is a plastic film.
 5. The secondary batteryaccording to claim 1, wherein the cushioning material is partly exposed.6. An electronic device comprising the secondary battery according toclaim 1, wherein the cushioning material is a plastic film, and whereinan area of the plastic film is larger than an area of a region where thefirst current collector and the second current collector overlap witheach other.
 7. An electronic device comprising: the secondary batteryaccording to claim 1; and at least one of a display portion, a housing,a speaker, a microphone, and an operation button.
 8. A secondary batterycomprising: a first current collector, an active material layer, asecond current collector, a plurality of cushioning materials, and afilm, wherein the film surrounds the first current collector, the activematerial layer, the second current collector, and the plurality ofcushioning materials, and wherein the first and second currentcollectors are positioned between the plurality of cushioning materials.9. The secondary battery according to claim 8, further comprising aseparator between a surface of the first current collector and a surfaceof the second current collector, wherein each of the plurality ofcushioning materials has a larger thickness than the separator.
 10. Thesecondary battery according to claim 8, wherein each of the plurality ofcushioning materials is an elastic body.
 11. The secondary batteryaccording to claim 8, wherein each of the plurality of cushioningmaterials is a plastic film.
 12. The secondary battery according toclaim 8, wherein each of the plurality of cushioning materials is partlyexposed.
 13. An electronic device comprising: the secondary batteryaccording to claim 8; and at least one of a display portion, a housing,a speaker, a microphone, and an operation button.